Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA

[Linus Walleij]

Hi Jim,

thanks for your patch!

On Tue, Sep 26, 2023 at 7:52 PM Jim Quinlan <james.quinlan@xxxxxxxxxxxx> wrote:

> Without this commit, the use of dma_alloc_coherent() while
> using CONFIG_DMA_RESTRICTED_POOL=y breaks devices from working.
> For example, the common Wifi 7260 chip (iwlwifi) works fine
> on arm64 with restricted memory but not on arm, unless this
> commit is applied.
>
> Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>

(...)
> +       select DMA_DIRECT_REMAP

Christoph invented that symbol so he can certainly
explain what is missing to use this on ARM.

This looks weird to me, because:
> git grep atomic_pool_init
arch/arm/mm/dma-mapping.c:static int __init atomic_pool_init(void)
kernel/dma/pool.c:static int __init dma_atomic_pool_init(void)

Now you have two atomic DMA pools in the kernel,
and a lot more than that is duplicated. I'm amazed that it
compiles at all.

Clearly if you want to do this, surely the ARM-specific
arch/arm/mm/dma-mapping.c and arch/arm/mm/dma-mapping-nommu.c
needs to be removed at the same time?

However I don't think it's that simple, because Christoph would surely
had done this a long time ago if it was that simple.

Yours,
Linus Walleij